This invention relates to the art of controlling hydraulic fluid flow rates within a hydraulic system in proportion to a predetermined volume of flow within the system, and particularly the control of the rate of hydraulic fluid flow in an hydraulic aircraft landing gear extension and retraction system depending upon the position of the landing gear in the extension and retraction cycles.
As a part of the design of hydraulic aircraft landing gear extension and retraction systems, the rate at which the system operates is generally established by the design characteristics of the hydraulic landing gear actuator mechanism in combination with the capabilities of the hydraulic system which operates the actuator mechanism. In order to vary the rate at which the landing gear extends or retracts at various different points in the operational cycle, it is generally necessary to design the actuator in such a manner that the rate of extension or retraction at various points in the cycle is a fixed function of actuator design. Thus, once the actuator is designed, it has not been possible to change the rate of landing gear extension or retraction at any stage in the operational cycle without either affecting the overall rate of operation of the landing gear extension and retraction throughout the cycle, or without redesigning the actuator itself to provide a changed rate of operation at the desired stage in the landing gear extension and retraction without affecting the rate of operation at other stages.
The significance of being able to vary the rate of system operation at different points in the cycle independently of affecting system operation at other points in the cycle is that the factors which regulate acceptable rates of operation are different at various stages in the operation of the system. For example, a primary requirement of system operation is speed in order to assure rapid landing gear retraction and extension to meet aircraft performance requirements. Conversely, at other points in the operational cycle, the speed of operation must be slowed to avoid excessive noise or potential damage to the landing gear system and to the aircraft as the landing gear approach the fully retracted or fully extended positions.
Further, depending upon design changes in the landing gear system, the points in the operational cycle at which the rate of system operation must be regulated and the specific rates of operation which are acceptable may change. For example, if the weight of the landing gear is increased or decreased, then the point at which the rate of landing gear extension or retraction should be slowed will also change in order to assure that the heavier or lighter landing gear will retract wihout imposing excessive mechanical loads on the system, while at the same time assuring sufficient rate to assure smooth landing gear operation uninterrupted by aerodynamic loads.
Thus, it appears that the need existed for a method or apparatus for independently regulating the rate of system operation at varing and different stages in the operational cycle, without the need for expensive redesign of the landing gear actuator mechanism whenever a change was made in other parts of the landing gear system. Additionally, because of the varying aerodynamic loads imposed on the system during its operational cycle, it also would be necessary to provide a method and apparatus which would uniformly and consistently regulate such rates of operation as a function of the specific position of the landing gear in the operational cycle, without being affected by varying aerodynamic loads which might cause differing retraction or extension times under varying circumstances.